1. Field of the Invention
The present invention relates to inorganic pigments, and more particularly to the production of finely divided inorganic pigments having improved wetting characteristics.
2. Description of the Prior Art
As known in the art finely divided inorganic pigments, such as hydrated silicas, alumina, calcium silicates, sodium alumino silicates and the like are used in many and widely varying applications. Such pigments are used, for example, as in the manufacture of paper, as reinforcing agents in rubber, as fillers in paints, in adhesives, etc.
For many years such pigments have been used by the ultimate consumer or processor in the form of an aqueous slurry or dispersion of the pigments. In recent years, significant innovations have been developed for stabilizing aqueous slurries of inorganic pigments and to prevent the pigments from settling and forming a cake or heel in the bottom of the storage tank, tank car or in other equipment employed by the processor. An example of such a technique for stabilizing aqueous slurries of inorganic pigments is disclosed in U.S. Pat. No. 3,291,626.
A further and very difficult problem remains however. This lies in the formation of the aqueous pigment slurry itself. In this regard, the general procedure for preparing a pigment slurry is to pump a given amount of water into the tank car, tank, or other vessel which contains a given weight of the pigment to make up a slurry having a specific concentration of solids. Because of practical and economical considerations pigment slurries generally have a solids content on the order of 10 to 50% by weight. To increase the wetting and the dispersion of the pigment into the aqueous medium prior art workers have used water at a relatively high temperature and thereafter allowed the slurry or resulting mixture to soak until the pigment is thoroughly dispersed. In addition, the mixture or dispersion has been agitated by bubbling air, or gas, through spargers or the like built into the tank car, tank or other vessel.
However, it has been found that even when hot water is employed, long soaking periods on the order of from about 12 to 24 hours are required. This is, of course, highly uneconomical and impractical. It has also been found that agitation of the mixture has little effect and is clearly not an adequate solution to the problem.
To improve the wetting of inorganic pigments, it has also been proposed to increase the moisture content of the pigment itself prior to its introduction or admixture with the aqueous medium. While this step improves wetting, it has been found that dispersion is still poor and that long soak times, as discussed above, are still required.